The present invention relates to medical devices and methods. More specifically, the invention relates to intervertebral prosthetic discs and methods of preserving motion upon removal of an intervertebral disc.
Back pain takes an enormous toll on the health and productivity of people around the world. According to the American Academy of Orthopedic Surgeons, approximately 80 percent of Americans will experience back pain at some time in their life. In the year 2000, approximately 26 million visits were made to physicians' offices due to back problems in the United States. On any one day, it is estimated that 5% of the working population in America is disabled by back pain.
One common cause of back pain is injury, degeneration and/or dysfunction of one or more intervertebral discs. Intervertebral discs are the soft tissue structures located between each of the thirty-three vertebral bones that make up the vertebral (spinal) column. Essentially, the discs allow the vertebrae to move relative to one another. The vertebral column and discs are vital anatomical structures, in that they form a central axis that supports the head and torso, allow for movement of the back, and protect the spinal cord, which passes through the vertebrae in proximity to the discs.
Discs often become damaged due to wear and tear or acute injury. For example, discs may bulge (herniate), tear, rupture, degenerate or the like. A bulging disc may press against the spinal cord or a nerve exiting the spinal cord, causing “radicular” pain (pain in one or more extremities caused by impingement of a nerve root). Degeneration or other damage to a disc may cause a loss of “disc height,” meaning that the natural space between two vertebrae decreases. Decreased disc height may cause a disc to bulge, facet loads to increase, two vertebrae to rub together in an unnatural way and/or increased pressure on certain parts of the vertebrae and/or nerve roots, thus causing pain. In general, chronic and acute damage to intervertebral discs is a common source of back related pain and loss of mobility.
When one or more damaged intervertebral disc cause a patient pain and discomfort, surgery is often required. Traditionally, surgical procedures for treating intervertebral discs have involved discectomy (partial or total removal of a disc), with or without interbody fusion of the two vertebrae adjacent to the disc. When the disc is partially or completely removed, it is necessary to replace the excised disc material with natural bone or artificial support structures to prevent direct contact between hard bony surfaces of adjacent vertebrae. Oftentimes, pins, rods, screws, cages and/or the like are inserted between the vertebrae to act as support structures to hold the vertebrae and any graft material in place while the bones permanently fuse together.
A more recent alternative to traditional fusion is total disc replacement or TDR. TDR provides the ability to treat disc related pain without fusion provided by bridging bone, by using a movable, implantable, artificial intervertebral disc (or “disc prosthesis”) between two vertebrae. A number of different artificial intervertebral discs are currently being developed. For example, U.S. Pat. Nos. 7,442,211; 7,531,001 and 7,753,956 and U.S. Patent Application Publication Nos. 2007/0282449; 2009/0234458; 2009/0276051; 2010/0016972 and 2010/0016973 which are hereby incorporated by reference in their entirety, describe artificial intervertebral discs with mobile bearing designs. Other examples of intervertebral disc prostheses are the Charité® disc (provided by DePuy Spine, Inc.) MOBIDISC® (provided by LDR Medical (www.ldrmedical.fr)), the BRYAN Cervical Disc (provided by Medtronic Sofamor Danek, Inc.), the PRODISC® or PRODISC-C® (from Synthes Stratec, Inc.), the PCM disc (provided by Cervitech, Inc.), and the MAVERICK® disc (provided by Medtronic Sofomor Danek).
These known artificial intervertebral discs generally include superior and inferior endplates which locate against and engage the adjacent vertebral bodies, and a core for providing motion between the plates. The core may be movable or fixed, metallic, ceramic or polymer and generally has at least one convex outer surface which mates with a concave recess on one of the plates in a fixed core device or both of the plates for a movable core device. In order to implant these intervertebral discs, the natural disc is removed and the vertebrae are distracted or forced apart in order to fit the artificial disc in place. Depending on the size of the disc space, many of the known artificial discs have a height which is higher than desired resulting in an unnaturally over distracted condition upon implantation of the disc. For example, a smallest height disc available can be about 10-13 mm for lumbar discs and about 5-6 mm for cervical discs.
Currently available artificial intervertebral discs do not provide a desired low profile for some patients with smaller disc heights. It would be desirable to provide a lower height disc which mimics more closely the natural anatomy for smaller patients.
In addition, the vertebral body contacting surfaces of many of the known artificial discs are flat. The inventor has recognized that this flat configuration does not generally match the surfaces of the vertebral bodies resulting in less than ideal bone to implant contact surfaces. It would be desirable to provide a more anatomically shaped vertebral body contacting surface for an artificial disc.
According to an aspect of the present invention, an intervertebral disc comprises: a superior endplate having an upper vertebra contacting surface and a lower bearing surface, wherein the upper surface of the upper endplate has a domed central portion and wherein the lower bearing surface has a concavity disposed opposite the domed central portion; an inferior endplate having a lower vertebra contacting surface and an upper surface, wherein the lower surface of the lower endplate has a domed central portion and wherein the upper bearing surface has a concavity disposed opposite the domed central portion; a core positioned between the superior and inferior endplates, the core having upper and lower surfaces configured to mate with the bearing surfaces of the superior and inferior endplates; and wherein the domed central portion of the superior endplate has a height greater than a height of the domed central portion of the inferior endplate.
According to another aspect of the present invention, an intervertebral disc comprises: a superior endplate having an upper vertebral contacting surface and a lower bearing surface, wherein the upper vertebral contacting surface of the superior endplate has a central portion that is raised relative to a peripheral portion of the superior endplate, and wherein the lower bearing surface has a concavity disposed opposite the raised central portion; an inferior endplate having a lower vertebral contacting surface and an upper surface, wherein the lower vertebral contacting surface of the inferior endplate has a central portion that is raised relative to a peripheral portion of the inferior endplate and wherein the upper bearing surface has a concavity disposed opposite the central portion; and a core positioned between the upper and inferior endplates, the core having upper and lower core bearing surfaces configured to mate with the bearing surfaces of the upper and inferior endplates; wherein the upper vertebral contacting surface of the superior endplate has a different shape than the lower vertebral contacting surface of the inferior endplate.